Cenozoic orogenic evolution of the Southern Central Andes (32-36°S)

Abstract

This review explores the complex interactions of endogenic and exogenic processes in the segment of the Andes that straddle a transition from flat slab to normal subduction (32°-36°S). This segment shows remarkable along-strike variations in topographic uplift, structural elevation, amount and rate of shortening, and crustal root geometry. In the flat-slab segment, high elevations, several tectonic provinces and the lack of active volcanism characterize the orogen. Deformation and uplift advanced to the east, together with arc-related magmatic activity, sequentially uplifting the Principal Cordillera (20 to ~8 Ma), the Frontal Cordillera (12 to 5 Ma), the Precordillera (<10 Ma) and the Sierras Pampeanas (<5 Ma). In the normal subduction segment, the Andes are characterized by a decrease in elevation, with a big step in topography at ~35°S and the development of an active magmatic arc straddling the Argentina-Chile border. The Frontal Cordillera is only in the northern part of normal subduction segment, disappearing at 34ºS; south of this latitude, only the Principal Cordillera remains. Deformation progressively advanced to the east, uplifting the Principal Cordillera (20 to 8 Ma), the Frontal Cordillera (<10 Ma) and the San Rafael basement block (<5 Ma). The amount of shortening systematically decreases from north to south, but at the transitional zone between flat and normal subduction segments, there is a sharp decline from ~180 km of shortening (32°S) to ~70 km (33°40´S). South from this latitude, the amount of shortening lineally decreases until it reaches ~30 km at 35°S. Yet, interestingly, the amount of late Miocene surface uplift is opposite that of the trend in crustal shortening. These along-strike variations are best explained by boundary conditions of the subduction system related to interplate dynamics controlling the overall pattern of tectonic shortening. However, local variations in mean topographic elevation, deformation styles and crustal root geometry are more likely to be due upper-plate lithospheric strength variations. These strength variations govern the degree of coupling between brittle upper crust and ductile lower crust deformation. In the flat-slab segment, an initial thick and felsic crust favors the coupling model; while in the normal subduction segment, a thin and mafic lower crust allows the uncoupling model.